1. Field of the Invention
The present invention relates to a pattern inspection apparatus and a method, and more particularly to a pattern inspection apparatus and a method of inspecting a fine pattern, such as a semiconductor integrated circuit (LSI), a liquid crystal panel, and a photomask (reticle) for the semiconductor or the liquid crystal panel, which are fabricated based on design data.
2. Description of the Related Art
For the pattern inspection of a wafer in fabricating process of semiconductor integrated circuit or the pattern inspection of a photomask for pattern formation thereof, an optical pattern inspection apparatus that uses the die-to-die comparison method has been used. In the die-to-die comparison method, a defect is detected by comparing an image obtained from a die to-be-inspected and an image obtained from the equivalent position of a die adjacent to the die to-be-inspected. In this case, the die means a semiconductor device.
On the other hand, for the inspection of a photomask (reticle) where no adjacent die exists, a die-to-database comparison method has been used. In the die-to-database comparison method, mask data is converted into an image. Then the image is used for a substitution of the image of the adjacent dies, and inspection is performed in the same manner as the above. Here, the mask data is data obtained by applying photomask correction to the design data. The technology concerned is disclosed, for example, in U.S. Pat. No. 5,563,702, “Automated photomask inspection apparatus and method.”
However, by using the die-to-database comparison method for wafer inspection, corner roundness of a pattern formed on a wafer or the like is likely to be detected as a defect. In order to solve this problem, a pretreatment, which adds corner roundness to the image converted from the mask data by applying a smoothing filter, has been used. However, corner roundness added by the pretreatment may be different from corner roundness of each pattern actually formed on the wafer. Therefore, an allowable deformation quantity should be set in order to ignore the above difference. As a result, a problem in which a fine defect locating in a place except a corner cannot be detected has happened.
The above problem is not severe for the die-to-database comparison photomask inspection, because the photomask should correspond to the mask data as much as possible. Thus, presently, the die-to-database comparison photomask inspection has put into practical use. However, the above problem is severe for the die-to-database comparison wafer inspection, because a pattern formed on the wafer is allowed to be deformed as long as an electrical characteristic is guaranteed. This allowable deformation quantity is considerably large. Actually, pattern deformation occurs due to a difference of stepper exposure condition, or the like. Therefore, the die-to-database comparison wafer inspection has not been put into practical use.
From a viewpoint of problems in semiconductor integrated circuit fabrication, a repeated defect (systematic defect) is more important issue than a random defect caused by a particle or the like. The repeated defect is defined as a defect that occurs repeatedly over all the dies on the wafer caused by the photomask failure, or the like. Because the repeated defects occur in the die to-be-inspected and in the adjacent dies that are to be compared with the die to-be-inspected, the die-to-die comparison wafer inspection cannot detect the repeated defects. Therefore, the die-to-database comparison wafer inspection has been demanded.
Although it has not been put into practical use because of calculation cost or the like, there has been proposed an inspection method using the design data and the wafer image. This inspection method is disclosed in, for example, a literature: “Automatic failure part tracing method of a logic LSI using an electron beam tester,” NEC Technical Report, vol. 50, No. 6, 1997. In this literature, the following methods are disclosed: a method using a projection of wiring edges on the X- and Y-axes; a method in which wiring corners are focused on; and a method in which a genetic algorithm is applied. Moreover, as a method used in this literature, a matching method in which after edges are approximated by straight lines, closed areas are extracted, and the closed areas are used for inspection is described. However, these methods fail to realize an inspection speed that is usable in high-speed inspection, and fail to perform the matching while detecting a pattern deformation quantity.
Further, presently, the auto defect classification (ADC) method using an image of a die having a defect has been used. However, the method cannot classify whether a killer defect or not, because it is not recognized which part of a circuit is destroyed by the defect.
Moreover, position of a defect detected by the die-to-die comparison inspection has an error caused by precision of a stage and an optical system of an inspection apparatus, and such error is approximately ten or more times larger than a wiring pattern. Due to the error, even if a defect position is related with the design data, relationship between the defect position and the design data cannot be recognized.
Recently, a line width of semiconductor integrated circuits is far shorter than wavelength used in a lithography process. In the lithography process, a method of adding OPC (Optical Proximity Correction) patterns has been used. In the method, by using a photomask fabricated by mask data that is created by adding OPC pattern to the design data, a pattern formed on a wafer fabricated by the photomask can be consistent with the design data as much as possible. Adding OPC patterns is one of the most important techniques for photomask correction.
If the OPC pattern does not effectively correct a pattern formed on a wafer, repeated defect occurs. However, the die-to-die comparison wafer inspection cannot detect the repeated defect. In order to solve this problem, it is necessary to provide a method in which the pattern formed on the wafer is inspected based on the design data with considering an allowable pattern deformation quantity.
In addition, in a multi-product/small-volume fabricating process, e.g. a system-on-a-chip (SoC) fabricating process, a short delivery time is required. In the fabricating process, when a repeated defect is detected in electric inspection as a final inspection, a short delivery time cannot be achieved. In order to solve this problem, it is necessary to provide an inspection method that inspects a difference between a pattern formed on a wafer and the design data for each lithography process. In the inspection method, it is required that an allowable pattern deformation quantity that does not affect an electrical characteristic is set, and a deformation quantity that exceeds the allowable deformation quantity should be detected.
Further, a lithography simulator checks the design data and an OPC pattern in order to evaluate the OPC pattern. Although the entire device can be verified by the lithography simulator, a simulated pattern cannot be necessarily the same as an actual pattern. Moreover, a defect except for a defect caused by the OPC pattern cannot be detected. A random defect existing on a photomask, a stepper aberration, or the like is an example of the defect.
Moreover, for verifying the simulation, it is necessary to provide a method in which a simulation pattern outputted from the lithography simulator is verified with an image of the pattern actually formed on the wafer. Moreover, it becomes increasingly important to improve the technology for circuit design by setting the allowable deformation quantity to the design data precisely and in detail.
A CD-SEM (Critical Dimension Scanning Electron Microscope) has been used for controlling a line width of a wafer in a fabricating process of semiconductor integrated circuits. The CD-SEM automatically measures a line width of a line-shaped pattern in a specified position using a line profile. Several positions in several shots on several pieces of the wafers for each lot are measured in order to control stepper exposure condition by using the CD-SEM.
As control items of the circuit pattern, end shrinkage of a wiring, a position of an isolated pattern, and the like are also important besides the line width, but the automatic measuring function of the CD-SEM allows only one-dimensional measurement. Specifically, the CD-SEM can measure only a length such as a line width. Thus, those two-dimensional shapes are inspected by an operator using an image obtained by the CD-SEM or other microscopes manually.
Generally, OPC pattern plays an important role not only to guarantee a gate line width, but also to form shapes of a corner and an isolated pattern. Furthermore, because of improvement of a processor frequency, control of a shape of an end or base of a gate pattern, which is called an end-cap or a field extension, respectively, also becomes important in addition to the gate line width.
The above inspections of two-dimensional patterns are essential both in sampling inspection in a fabricating process and in a R&D fabricating process. Especially, in the R&D fabricating process, it is necessary to inspect all patterns formed on the wafer. However, in a present situation, the control of the two-dimensional shape is performed by a human work, and is not perfectly performed. In order to solve this problem, automated die-to-database comparison wafer inspection is required.
As concrete subjects for automatization, the following subjects are enumerated:
1. In order to detect repeated defects in each semiconductor device, it is practically difficult to check whether there are defects at the same location by comparing huge defect information.
2. In order to control a semiconductor device process, it is necessary to provide automatic inspection of a line width of a line-shaped pattern, an average line width of a line-shaped pattern, a space width of a line-shaped pattern, and an average space width of a line-shaped pattern. Although complex calculation is required, it is also necessary to provide automatic inspection of a line width of a curvilinear-shaped pattern such as a corner part, an average line width of a curvilinear-shaped pattern, a space width of a curvilinear-shaped pattern, and an average space width of a curvilinear-shaped pattern.
Especially, the inspection of all the gate widths of the entire semiconductor is important for performance improvement. However, huge labor cost is required, because it is necessary that overlapping part of a polycrystalline silicon layer and an active layer (preceding process layer of the polycrystalline silicon layer) should be extracted.
In addition, in order to obtain information as to an open or bridge, which is more important than other defects, as a kind of defect-class, it is necessary to inspect an image again by an operator. Further, it is necessary to strengthen recognition capability of the open or bridge defect that is observed as a weak contrast.
As other request, it is necessary to shorten an image-acquiring time by using a method in which only parts corresponding to areas for the above inspection are scanned.
3. It is necessary to provide an inspection method of controlling a contact-area of a contact hole/via hole with an end of a wiring layer. In the method, an allowable deformation quantity for controlling shrinkage of the end should be automatically determined by judging whether the end and the contact hole/via hole have enough margin or not. Moreover, it is necessary to provide an inspection method that uses a contact-area as an evaluation value.
4. A correction pattern as a kind of OPC pattern is added to mask data in order to correct a pattern. The correction pattern is located adjacent to the pattern to-be-inspected, and should not be formed on a wafer. However, the correction pattern might be formed as a defect. A method of inspecting such defect is required.
5. It is necessary for a pattern inspection apparatus to have a method of converting a huge inspection result into information suitable for evaluation of a pattern deformation quantity in an entire semiconductor device or evaluation of stepper aberration or the like, and a method of displaying the converted information.
6. A rotation of a specimen caused by stage moving or the like may cause a rotation of a pattern image to-be-inspected. Moreover, the electrification phenomenon of the specimen or the like may cause deformation such as a rotation including a skew or a variation of magnification or the like. Because of the above image distortion, a smaller defect than the above distortion quantities cannot be detected. The above distortion occurs sporadically, and the distortion cannot be predicted. Therefore, it is necessary to detect the above distortion quantities, and correct the above image distortion every time when an image is acquired.
7. It is necessary that methods of correcting image distortions held by the image generation device generating an image with the large field of view can be provided. The image distortions include a nonlinear image distortion and a variation of a line width depending on a position in the image. It is desired that the correction methods can perform automatically and accurately in a short time.
8. Because results of defect inspection are huge, it is necessary for a pattern inspection apparatus to have a method of reducing the number of defects and detecting important defects reliably by dividing a pattern deformation quantity into a global deformation quantity and a local deformation quantity.
9. In the case of long-term inspection, a beam spot size may be varied gradually. The wider beam spot size is, the wider measurement values of a line width is. Thus, it is necessary to correct a variation of a measurement value of a line width caused by the variation of the beam spot size.
10. In order to recognize tendencies for defects to be generated, it is necessary to classify huge defects using defect-classes that are determined by geometrical information of a reference pattern, information of the design data, or information of data related to the design data.
11. If many of the defects belong to the same defect-class and one or some of the defects belong to another or other defect-classes, an image or some images of the latter defect(s) cannot be registered.
12. The defects having the same defect-class that are occasionally generated are reinspected less adequately than the defects having the same defect-class that are frequently generated.
13. It is necessary to recognize tendencies for defects to be generated easily by separating defects into defects generated in thin and dense parts and defects generated in rough parts.
14. If a distance between two line segments that face each other closest together is shorter than a predetermined distance, the two line segments require signal intensity correction for canceling effects of phenomena caused by a variation of generation rate and capture rate of secondary charged particles. In addition, if a distance between two line segments that face each other closest together is longer than another predetermined distance, the two line segments require signal intensity correction. Therefore, it is necessary to correct the signal intensity.
15. A region suitable for image adjustment has been determined experimentally by an operator. There has been a demand for automatic extraction method of recognizing a region suitable for image adjustment using geometrical information of line segments of the design data or using relationship between line segments of the design data that are connected or are located closely. Further, if part of an image is used for image adjustment, automatic adjustment may be performed more accurately. However, conventionally, there has been only a method of using the whole image.
16. The design data is mostly composed of horizontal lines and vertical lines. By using this characteristic of the design data, it is necessary to perform matching at high speed using projection data obtained by projecting an edge of a reference pattern on the horizontal and vertical axes, and projection data obtained by projecting an edge of an image of a pattern to-be-inspected on the horizontal and vertical axes.
17. It is difficult to perform matching of a boundary between a region where the same patterns are periodically arranged and other regions. Thus, it is necessary to solve this problem.
18. A calculation time for matching of hole patterns or island patterns is longer than a calculation time for matching of line-shaped patterns, because the hole patterns or the island patterns are smaller and more numerous than line-shaped patterns. In order to solve this problem, it is necessary to provide a method of calculating at high speed.
19. Image brightness of hole patterns or island patterns may be non-uniform depending on an image position due to electrification phenomenon and the like. Thus, it is necessary to provide a matching method that can perform matching even if the image brightness of the hole pattern or the island pattern is non-uniform.
20. In the case where an inspection-unit-area is divided into sub-inspection-unit-areas, it is necessary to provide a method in which matching is performed by using a sub-inspection-unit-area which is the most suitable for matching.
21. In the case where there is a pattern of a lower layer formed in the preceding process beneath a pattern to-be-inspected, part of a pattern to-be-inspected where there is a pattern of the lower layer formed in the preceding process and part of a pattern to-be-inspected where there is no pattern of the lower layer formed in the preceding process have different shapes, and are sometimes observed differently. In order to solve this problem, it is necessary to provide a method in which inspection is performed by using different inspection parameters according to part of the pattern to-be-inspected where there is a pattern of the lower layer formed in the preceding process and part of the pattern to-be-inspected where there is no pattern of the lower layer formed in the preceding process.
22. If one of the design data, mask data (data created by adding OPC pattern to the design data), a feature obtained from simulation using the design data, and data related to the design data is related to the inspection result, and the related information and the inspection result are displayed together, it becomes easy to recognize tendencies for defects to be generated. Therefore, it is necessary to provide a method of displaying the related information and the inspection result together.
23. In a lithography process, patterns formed in a resist film on a silicon substrate are inspected. In this case, if the patterns formed in the resist film are inspected using an electron beam (charged particle beam), the electrification phenomenon occurs, because the resist is generally composed of high polymer which is insulation. Therefore, a shape of the pattern in a pattern image to-be-inspected is deformed by the electrification phenomenon, because the electron beam is deflected by a top surface of the resist film that is partially charged, and the electron beam is applied to an inaccurate position. Therefore, it is necessary to prevent the electrification phenomenon from occurring to obtain a pattern image having no deformation.
24. There has been practiced a process of using a scanning electron microscope and making automatic measurements by keeping all scanning directions for an electron beam (charged particle beam) constant in the scanning electron microscope. However, the process is problematic in that it causes a measurement error depending on the direction of beam segments. Therefore, it is necessary to provide a method that can automatically set various measuring conditions based on reference data by utilizing a reference pattern.
The present invention has been made in view of the above problems. Therefore, it is an object of the present invention to provide a pattern inspection apparatus and method, which use information of the design data.